1. Field of the Invention
The present invention relates to a superimposing circuit module, and more particularly, to a superimposing circuit module used as a high-frequency superimposing circuit for use in conjunction with a laser diode in an optical pickup for reading or recording/reading information on/from an information storage medium such as a magnetooptical disk.
2. Description of the Related Art
In an optical pickup for recording/reading information on/from a magnetooptical storage medium, information is read by detecting light reflected from a magnetooptical disk illuminated with a light beam, on the basis of the Kerr effect. However, in the reading of information from a magnetooptical disk, if light returns from the magnetooptical disk to an end surface of a laser diode, the reproduced signal is modulated by the returned light. To avoid this problem, the optical pickup includes a high frequency superimposing circuit having a high frequency oscillator for canceling out the effect of the light returned to the end surface of the laser diode.
FIG. 2 is a cross-sectional view illustrating the structure of a conventional superimposing circuit module 31 functioning as a high frequency superimposing circuit. In the conventional high frequency superimposing circuit, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 7-93758 or 7-105561, chip-shaped components 36 such as transistors, capacitors, resistors, and inductors are mounted on both sides of a printed circuit board 35, and the printed circuit board 35 is placed in a case 32 such that it is positioned by a protrusion 34. The case 32 is covered with a cover 33. Leads 37a, 37b, and 37c of a laser diode 25 disposed on the bottom surface of the holder case 32 are passed through the printed circuit board 35 and soldered to interconnection patterns on the printed circuit board 35. Furthermore, three terminals pins 38a, 38b, and 38c, which extend through the printed circuit board 35 and are connected to interconnection patterns on the printed circuit board 35, are arranged to extend through a feedthrough capacitor 39 disposed on the bottom inner surface of the case 32 and are extended to the outside from the bottom surface of the case 32a. 
In the conventional superimposing circuit module, as shown in FIG. 2, a large area is needed to mount chip-shaped components on both sides of the printed circuit board to form the circuit including the oscillator. As a result, the size of the printed circuit board becomes large and thus, the total size of the superimposing circuit module becomes large.
As the size of the superimposing circuit module increases, the length of the interconnection patterns disposed on the surfaces of the printed circuit board increases. The increase in the length of the interconnection patterns results in an increase in undesired spurious signal emission. To block the spurious signal emission, it is necessary to entirely shield the printed circuit board with the case and the cover.
If the size becomes even larger, the result is further increases in cost and the number of production steps because the substrate must be entirely covered with the case and the cover and besides because the feedthrough capacitor must be mounted.
Furthermore, in the above-described superimposing circuit module, because discrete components are mounted on the printed circuit board, circuit parameters of the respective circuit components such as inductors and capacitors (in particular, parameters which determine the resonance frequency of the resonance circuit) should be carefully selected to achieve matching among these circuit components. The process of accurately determining the circuit parameters requires a long time.
In order to overcome the problems described above, preferred embodiments of the present invention provide a small-sized superimposed circuit module having reduced spurious electromagnetic wave emission and having small variations in the circuit parameters.
According to a preferred embodiment of the present invention, a superimposing circuit module is arranged to minimize the effect of returned light upon a laser light source for illuminating an information storage medium with a laser beam, the superimposing circuit module including a multiplayer substrate and a portion or all of the circuit elements of an oscillator in the superimposing circuit are embedded in the multilayer substrate. In particular, it is desirable that, of the circuit elements defining the oscillator, some or all of a resonance circuit including a capacitor and an inductor or including a stripline, are embedded in the multilayer substrate.
In this superimposing circuit module according to this preferred embodiment of the present invention, because some or all of the circuit elements of the oscillator in the superimposing circuit are embedded in the multilayer substrate, the size of the oscillator is greatly reduced and thus the total size of the superimposing circuit module is greatly reduced.
The reduction in the size of the oscillator and the superimposing circuit module results in a reduction in the length of the interconnection pattern provided on the surface of the substrate, which in turn results in a reduction in spurious signal emission.
Furthermore, the multilayer structure and the small size of the oscillator allow reductions in the number of required manufacturing steps and manufacturing cost.
Furthermore, in this superimposing circuit module, because the circuit elements of the oscillator can be formed by printing electrodes or interconnection conductors, the variations in the circuit parameters can be minimized and thus, the matching adjustment becomes unnecessary.
According to another preferred embodiment of the present invention, a superimposing circuit module arranged to reduce the effects of returned light upon a laser light source for illuminating an information storage medium with a laser beam is characterized in that some of or all of circuit elements of a noise rejection filter in the superimposing circuit are embedded in a multilayer substrate. In particular, it is desirable that, of the circuit elements defining the noise rejection filter, a capacitor, an inductor, or a stripline be embedded in the multilayer substrate.
In this superimposing circuit module according to preferred embodiments of the present invention, because some or all of circuit elements of the noise rejection filter are embedded in the multilayer substrate, the size of the noise rejection filter is greatly reduced and thus, the total size of the superimposing circuit module is greatly reduced.
The reduction in the size of the noise rejection filter and the superimposing circuit module results in a reduction in the length of the interconnection pattern disposed on the surface of the substrate, which in turn results in a reduction in spurious signal emission.
Furthermore, the multilayer structure and the small size of the noise rejection filter allow reductions in the number of manufacturing steps and manufacturing cost.
Furthermore, in this superimposing circuit module, because the circuit elements of the noise rejection filter can be formed by printing electrodes or interconnection conductors, the variations in the circuit parameters can be minimized and thus the matching adjustment becomes unnecessary.
In the superimposing circuit module according to a preferred embodiment of the present invention, it is desirable that an amplification device be mounted on the surface of the multilayer substrate. Although the amplification device such as a transistor is difficult to embed in the multilayer substrate, it is easy to mount the amplification device on the surface of the multilayer substrate. The mounting of the amplification device on the surface of the multilayer substrate makes it possible to construct the superimposing circuit module to have a simple structure.
In the superimposing circuit module according to various preferred embodiments of the present invention, it is desirable that the surface of the multilayer substrate is covered with a metal case.
Covering the surface of the multilayer substrate with the metal case allows for a further reduction in the spurious signal emission. Furthermore, because only the surface of the multilayer substrate is needed to be covered with the metal case, the size of the superimposing circuit module can be reduced and thus, the cost thereof can also be reduced.
In the superimposing circuit module according to various preferred embodiments of the present invention, it is desirable that the multilayer substrate be defined by laminating a copper electrode film and a low-temperature-baked ceramic layer.
If the multilayer substrate is formed by lamination of a copper electrode film and a low-temperature-baked ceramic layer, it becomes possible to embed, in the multilayer substrate, not only a capacitor plate and an inductor but also a shield plate for blocking an electromagnetic wave.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.